Copending U.S. patent application Ser. No. 580,532 (U.S. Pat. No. 4,585,618) (Canadian patent application No. 447,647 filed on Feb. 16, 1984) discloses the production of metal/ceramic composite bodies comprising materials as mentioned in the preamble, i.e. borides, nitrides, carbides or silicides of at least one of titanium, hafnium and zirconium, a metal and an oxide of said metal. The disclosed method of producing these composite bodies comprises the reaction sintering of a mixture of precursor powders containing an oxide of the transition metal, boron oxide (in the case of the desired resulting compound of the transition metal being a boride) and e.g. aluminum as a metallic component of the reaction mixture. The resulting metal/ceramic composite, which will be further referred to as "cermet" comprises e.g. aluminum as the metal component; a minor amount of which (less than 20%) may come from excess aluminum powder added to the precursor mixture, the major part or, in the case of no excess aluminum in the precursor, all of it comes from infiltration of liquid aluminum during the sintering which is carried out under liquid aluminum.
Other examples for such processes are described in U.S. application Nos. 454,669; 454,670 (U.S. Pat. No. 4,514,268); 454,671 (U.S. Pat. No. 4,605,634); 454,672 (U.S. Pat. No. 4,540,475); 454,673 (U.S. Pat. No. 4,605,633); 454,674 (U.S. Pat. No. 4,534,835) and 626,451 (U.S. Pat. No. 4,610,726); (Canadian application Nos. 440,745; 440,729 and 440,744; all filed on Nov. 8, 1983 in the name of Corning Glass Works). In these publications ceramic or ceramic-metal bodies are produced by admixing powders of TiH.sub.2 and AlB.sub.2, the reaction sintering yielding an intimately mixed composite body comprising TiB.sub.2 and Al, whereby H.sub.2 is evolved. In other described examples TiO.sub.2 or TiN together with metallic aluminum and boron is used to produce composite bodies of TiB.sub.2 and Al.sub.2 O.sub.3 or TiB.sub.2 and AlN respectively.
For a variety iof applications it is of great importance that the microstructure of the sintered body be sufficiently homogeneous to comply with the stringent requirements of the chemical and physical environment in which these articles are used, e.g. in fused-salt aluminum production cells which operate at temperatures near 1000.degree. C. in a highly reactive environment. Other uses of such materials may be envisaged in the automotive industry for structural parts of internal combustion engines or other machines, subject to mechanical and heat cycling stresses, or for cutting tools, where special properties like hardness are requested, which may be negatively affected by deficiencies in the microstructure.
The above U.S. patent application Ser. No. 580,532 (Canadian patent application No. 447,647) also reports that it may be advantageous to pre-sinter the pressed precursor body at temperatures below the temperature at which the reaction sintering is initiated. Compared with simply pressed precursor pellets which are very brittle and therefore difficult to handle, this pre-sintered bodies of unreacted but ductile precursors can be machined, stored or transported prior to the reaction sintering.
For particular applications of these known reaction sintered cermets such as for components in fused salt electrowinning cells, the contact of these cermets with molten metal requires in addition to a very homogeneous microstructure, a very fine grain size and a density which approaches the theoretical density.
The material according to the above U.S. patent application represents already a considerable improvement over other known materials, however, further optimization of the microstructural aspects of the material are desirable. Effects like grain boundary corrosion, erosion and so forth are more severe when agglomerations of grains of the same compound of a multi-compound composite body are built. It is therefore desirable, to produce a material, the respective grains of the different components of which are uniformly distributed without forming agglomerations of grains of the same nature.
The final microstructure may be influenced already by the choice of the particular precursor material, by operating parameters such as pressure, temperature, duration and slope of the heat treatment and so forth. However, a permanent, not yet resolved problem appears when articles of substantial dimensions are produced, since the ratio of the volume of the precursor powder to the volume of the pressed shape is in many cases relatively high and the pressing operation therefore already produces inhomogeneities throughout the pressed body. This drawback may in principle be overcome by isostatically pressing; however, isostatically pressing of large shapes which undergo a substantial reduction of volume is very difficult and expensive and yields shapes of irregular surfaces which have to be machined. Shapes of complicated structures are practically impossible to be isostatically pressed.
It is known to combine smaller pieces of ceramic or cermet bodies by assembling surfaces of the individual pieces to be joined with an intermediary precursor powder and reaction sintering to produce a joint which may be of the same material as the joined pieces or of a different one. In both cases it is impossible, however, to obtain a final microstructure which is homogeneous across the joint.